The present invention relates to an improvement of the liquid crystal device which operates at high speed and, more particularly, it relates to an improvement of the liquid crystal device which is driven according to the two-frequency addressing scheme.
The liquid crystal device is in use currently as the image display device for a television receiver, computer display and the like. It has also been employed by the so-called electrophotographic printer, as disclosed by U.S. Pat. No. 4,386,836, for example. This liquid crystal device which has been used for image display or with an electrophotographic printer has the following general arrangement in. A pair of substrates are arranged opposite to each other, sandwiching a liquid crystal material between them. A plurality of signal electrodes are provided on the inner face of one of the substrates, and a plurality of scanning electrodes are provided on the inner face of the other substrate, thereby forming a plurality of light shutters (microshutters) which serve to switch light on and off. These shutters comprise a part of that area where the signal and scanning electrodes are opposed to one another with the liquid crystal interposed between them. The liquid crystal material is sealed by a sealing member, which is formed along the outline of the paired substrates.
A liquid crystal device having the above-described arrangement may in some situations, need to be driven at high frequency to have high contrast. The two-frequency addressing scheme is used in this case. This two-frequency addressing scheme uses the dielectric dispersion phenomenon of liquid crystal. The two-frequency driving liquid crystal is used in the case of employing this two-frequency addressing scheme. This two-frequency driving liquid crystal has the property of changing its dielectric anisotropy positive and negative, depending upon the frequency of alternating electric field applied. As shown in FIG. 1, it is assumed that the frequency of an alternating electric field at which the dielectric anisotropy .DELTA..epsilon. of the liquid crystal becomes zero is fc. When an alternating electric field having a frequency fL lower than the frequency fC is applied, the liquid crystal material shows positive dielectric anisotropy .DELTA..epsilon.L and the molar axes of the liquid crystal become parallel to the electric field. When an alternating electric field having a frequency fH higher than the frequency fC is applied, the liquid crystal shows negative anisotropy .DELTA..epsilon.H and the molar axes of the liquid crystal become perpendicular to the electric field. As described above, the two-frequency addressing scheme is intended to selectively operate molecules of liquid crystal by selectively applying the alternating electric fields of low and high frequencies to the liquid crystal material. As a result, the amount of light passing through the shutters can be controlled.
A case where the two-frequency addressing scheme is applied to the liquid crystal device of the positive display TN type will be described. The liquid crystal device of the positive display TN type is of the twisted nematic mode wherein polarization axes of polarizer are arranged perpendicular to one another. When a voltage having the low frequency fL is applied between the signal and scanning electrodes, the molar axes of liquid crystal become perpendicular to the electrode face. The shutters thus shut off light and are turned off. When a voltage having the high frequency fH is applied between the signal and scanning electrodes, the molar axes of liquid crystal become twisted and parallel to the electrode face. The shutters thus allow light to pass therethrough and are turned on.
Another case where the two-frequency addressing scheme is applied to the liquid crystal device of the G-H mode (or Gest-Host effect mode) will be described. When the voltage of low frequency fL is applied between the electrodes, the molar axes of liquid crystal become perpendicular to the electrodes and the axes of dichroic dye which moves together with the molecules of liquid crystal also become perpendicular to the electrodes. Therefore, the dichroic dye absorbs no light and the shutters turn on, allowing light to pass therethrough. When the voltage of high frequency fH is applied between the electrodes, the molar axes of liquid crystal become parallel to the electrode face and those of dichroic dye thus become parallel to the electrode face, thereby absorbing light in a wavelength band particularly to the dye. Therefore, the shutters turn off, shutting off the light with this specific wavelength band.
In the above-described liquid crystal devices, the area of the portions which forms the plural microshutters among the electrodes is extremely small. The size of each of the shutters is 0.1 mm.times.0.1 mm, for example. However, the area of another portion except that one which forms the plural micro-shutters among the electrodes is large. The area of lead portion, for example, which serves to supply power to the shutters is large because the pattern width of the electrodes cannot be made narrower to keep the resistance value of the electrodes small. In addition, the liquid crystal material which has a large dielectric constant is interposed between the signal and scanning electrodes. Electrostatic capacitance which exists between both of these electrodes is thus extremely large. When a voltage having a high frequency is applied between both of these electrodes, therefore, large current is allowed to flow through the capacitance which is provided by both of these electrodes. On the other hand, the shape of the electrodes cannot be made so large from the viewpoint of space and without increasing the capacitance between both of the electrodes and it is therefore difficult to make the resistance value of the electrodes sufficiently small. Accordingly, the conventional liquid crystal devices which employed the two-frequency addressing scheme had such a drawback that their lead portions were heated. If the number of the signal electrodes is large, the amount of current flowing through the scanning electrodes is large even when the amount of current flowing through the signal electrodes is small. Therefore, the heat value was extremely large on the power side of the scanning electrodes.
Further, the liquid crystal material was deteriorated by the heat generated in the conventional liquid crystal devices. In addition, the operation characteristics of the liquid crystal material was changed by the heat generated. Particularly in the case where the liquid crystal devices are driven according to the two-frequency addressing scheme, the crossover frequency fC is changed to a great extent when the temperature of the liquid crystal material is raised. The dielectric anistropies .DELTA..epsilon.L and .DELTA..epsilon.H in relation to the low and high frequencies fL and fH are thus changed to a great extent, so that the liquid crystal devices cannot be driven stably. In addition, their response speed and contrast are reduced. The turning on and off of the shutters sometimes cannot be controlled.